Explicit end marker for models printed in the CVC language (#3934)

Fixes https://github.com/CVC4/cvc4-wishues/issues/9.

When communicating with CVC4 using pipes and the CVC language, it was not possible to determine when all the lines of a model have been printed.

This change adds begin and end markers as the example below:
```
MODEL BEGIN
x : INT = -3; 
y : INT = 0;  
z : INT = 0;
MODEL END;
```

1 files changed, 12 insertions, 0 deletions

diff --git a/test/regress/regress0/print_model.cvc b/test/regress/regress0/print_model.cvc
new file mode 100644
index 000000000..0ad9ddf4c
--- /dev/null
+++ b/test/regress/regress0/print_model.cvc
@@ -0,0 +1,12 @@
+% COMMAND-LINE: --produce-models
+% EXPECT: sat
+% EXPECT: MODEL BEGIN
+% EXPECT: s1 : INT = 2;
+% EXPECT: s2 : INT = 1;
+% EXPECT: MODEL END;
+
+OPTION "produce-models";
+s1, s2 : INT;
+ASSERT s1 = 2;
+CHECKSAT s2 > 0 AND s2 < s1;
+COUNTERMODEL;